This invention is directed to a process and apparatus for purifying hydrogen bromide by removing the impurities and trace metals.
Hydrogen bromide has a wide range of applications due to its ability to act as a catalyst and/or reagent. An important recent application in semiconductor fabrication is its use as an etchant. Generally, the main impurities in hydrogen bromide are oxygen, nitrogen, carbon monoxide, methane, hydrogen, moisture, carbon dioxide, hydrogen chloride and trace metals, such as iron. For good etching performance in semiconductor fabrication, it is generally necessary to have highly pure hydrogen bromide. Thus, purity levels of 99.999% are preferably to achieve control of the etching in fabricating semiconductor components.
In the table below, the 99.999% column shows the preferred required impurity levels of hydrogen bromide for use in semiconductor processes.
A number of prior art relating to hydrogen bromide is known, but none of which teaches or suggests the present invention.
U.S. Pat. No. 5,316,616 discloses a process of dry etching a material such as polycrystalline silicon and suicides with hydrogen bromide or bromine. Plasma etching with hydrogen bromide or bromine as an etching gas allows a precise control of attaining vertical etching or taper etching with a desired taper angle by controlling a temperature of a mass to be etched, which mass is a phosphorus-doped n-type poly-crystalline silicon, phosphorus-doped single crystalline or phosphorus-doped silicides semiconductor wafer.
U.S. Pat. No. 5,685,169 discloses a method and apparatus for preparing high purity hydrogen bromide. The starting material, hydrogen bromide, contains impurities having low boiling points is supplied to an intermediate space. While the gas phase of the starting hydrogen bromide is allowed to ascend through an upper rectifying section, it is brought into contact with a first reflux solution flowing in the reverse direction. The uncondensed gas stored in an upper space is cooled and partly condensed. The condensed liquid is allowed to flow down through an upper rectifying section as the first reflux solution. The liquid-phase of the starting hydrogen bromide is mixed with the first reflux solution in the intermediate space and serves as a second reflux solution. The liquid stored in a lower space is heated and partly evaporated. The liquid stored in the lower space is supplied outside as high purity hydrogen bromide. The uncondensed gas stored in the upper space is discharged outside.
U.S. Pat. No. 4,925,646 discloses a process for drying a gaseous hydrogen halide to remove water impurity therefrom, in which a scavenger precursor composition is provided, including a support having associated therewith partially or fully alkylated metal alkyl compounds or pendant groups. The precursor composition is reacted with gaseous hydrogen halide to convert the metal alkyl compounds and/or pendant functional groups to the corresponding metal halide compounds and/or pendant functional groups, which in turn react with the water impurity to produce an essentially completely water-free (below 0.1 ppm) gaseous hydrogen halide effluent.
U.S. Pat. No. 4,853,148 discloses a process for drying a gaseous hydrogen halide to remove water impurity therefrom. The active scavenging moiety is selected from one or more members of the group consisting of (i) metal halide compounds dispersed in the support, of the formula MXy; and (ii) metal halide pendant functional groups of the formula xe2x80x94MXyxe2x88x921 covalently bonded to the support, wherein M is a y-valent metal, and y is an integer whose value is from 1 to 3. The corresponding partially or fully alkylated compounds and/or pendant functional groups, of the metal halide compounds and/or functional groups of (a); wherein the alkylated compounds and/or pendant functional groups, when present, are reactive with the gaseous hydrogen halide to form the corresponding halide compounds and/or pendant functional groups of (a); and M being selected such that the heat of formation, xcex94Hƒ of its hydrated halide, MXy(H2O)n, is governed by the relationship xcex94Hƒxe2x89xa7nxc3x9710.1 kilocalories/mole of such hydrated halide compound wherein n is the number of water molecules bound to the metal halide in the metal halide hydrate.
U.S. Pat. No. 4,119,413 discloses a method for recovering hydrogen bromide gas from a gaseous stream with a nonaqueous hydrogen bromide absorbent containing acetic acid to absorb at least a portion of the hydrogen bromide. The hydrogen bromide containing absorbent is then heated to desorb at least a portion of the hydrogen bromide.
An object of the present invention is to provide a process for purifying hydrogen bromide comprising several steps in which the sequence of the steps produces a high purified hydrogen bromide.
Another object of the present invention is to provide a process for purifying hydrogen bromide comprising several steps in which one step involves the removal of the two acids, carbon dioxide and hydrogen chloride, from hydrogen bromide, which is another acid.
Another object of the present invention is to provide an economical and low cost process for purifying hydrogen bromide, which does not use the conventional distillation columns approach.
Another object of the present invention is to provide an apparatus for purifying hydrogen bromide.
The invention relates to a process for purifying hydrogen bromide comprising the steps of (a) feeding hydrogen bromide into a cooling bath to produce a flowable hydrogen bromide that is above its freezing temperature and below its ice water temperature; (b) feeding the hydrogen bromide from step (a) into a first adsorbent trap, such as a sand trap, to effectively remove moisture and any free bromine; (c) feeding the hydrogen bromide from step (b) into a molecular sieve to effectively remove any carbon dioxide and hydrogen chloride without removing the hydrogen bromide; and (d) feeding the hydrogen bromide from step (c) into a second adsorbent trap, such as a silica gel trap, to effectively remove any remaining moisture and trace metals to produce hydrogen bromide having a purity of above about 99.9% and preferably above about 99.999%. The purified hydrogen bromide of step (d) could be further filtered using a 0.01 micrometer filter to remove any particulates that may have been transported from the adsorbent traps and any trace metals that may be in the form of solids. The purified hydrogen bromide can then be accumulated in a vessel that is placed in a bath that is at a temperature between about xe2x88x9275xc2x0 C. and xe2x88x92196xc2x0 C., preferably between about xe2x88x9275xc2x0 C. and xe2x88x9285xc2x0 C., and most preferably about xe2x88x9278xc2x0 C. The temperature of the cooling bath in step (a) can be between about 0xc2x0 C. and xe2x88x9286.9xc2x0 C., preferably between about xe2x88x9210xc2x0 C. and xe2x88x9250xc2x0 C., and most preferably about xe2x88x9225xc2x0 C.
Generally, cylinders of hydrogen bromide are typically charged to a maximum fill density of 135%. This fill density is calculated by dividing the mass of the product in the cylinder by the mass of water the cylinder would contain when filled full. Consequently, a typical fill of hydrogen bromide would leave approximately 25% of the cylinder volume as vapor space. Thus, the hydrogen bromide cylinder generally is the storage vessel for crude hydrogen bromide to be purified. This hydrogen bromide is typically 99.0% pure and contains approximately 200 ppm oxygen, 3000 ppm nitrogen, 5000 ppm hydrogen, 200-300 ppm carbon monoxide, 200-300 ppm methane, 10-100 ppm carbon dioxide, 10 ppm moisture, 200 ppm hydrogen chloride and 5-10 ppm of various trace metals, mainly iron.
The head space of the crude hydrogen bromide can be vented by valve means or the like to remove oxygen, nitrogen, hydrogen, carbon monoxide and methane.
Consequently, an additional step could be added to the process before step (a) and such step would include venting the vapor in a cylinder containing hydrogen bromide to remove impurities of oxygen, nitrogen, hydrogen, carbon monoxide and methane. Preferably, the venting step could be repeated at intervals between 2 and 5 times for a time period of between 5 and 15 minutes to substantially remove the gas impurities recited above.
The subject invention also relates to an apparatus for purifying hydrogen bromide comprising a cooling container containing: (a) a cooling means, such as a cooling coil, adapted for cooling hydrogen bromide and having input and output openings; (b) a first adsorbent means with an input opening coupled to the output of the cooling means and an output opening, and the first adsorbent means being adapted for removal of moisture and free bromine from hydrogen bromide; (c) a molecular sieve means with an input opening coupled to the output of the first adsorbent means and an output opening, and the molecular sieve means being adapted for removing carbon dioxide and hydrogen chloride from hydrogen bromide; and (d) a second adsorbent means with an input opening coupled to the output of the molecular sieve means and an output opening, and second adsorbent means being adapted for removing moisture and trace metals from hydrogen bromide. A filter means could be added such that its input that could be coupled to the output of the second adsorbent means and an output opening and the filter means being adapted for removing any particulates from the hydrogen bromide that may have been transported from the adsorbent means of the apparatus.
Briefly, the present invention preferably involves a series in a particular sequence of a sand bed, a molecular sieve bed, and a silica gel bed. The primary contaminants of the hydrogen bromide process are hydrogen chloride and carbon dioxide. An unexpected finding was that a 5A molecular sieve trap would work for separating hydrogen chloride and carbon dioxide from hydrogen bromide. Removing hydrogen chloride and carbon dioxide (two acids) from a third acid, hydrogen bromide, is a very difficult separation. Discovering that 5A sieve would take three acids (hydrogen bromide, hydrogen chloride and carbon dioxide) with the same acidic properties and remove two of these acids (hydrogen chloride and carbon dioxide) without removing the hydrogen bromide was unexpected.
The preferred embodiment of the subject invention is a process based on head space rejection from a hydrogen bromide cylinder, a cooling coil, a sequence of sand, molecular sieve and silica gel traps, and filter and condensing means that will produce hydrogen bromide of 99.9% purity or higher.